1. Field of the Invention
The present invention relates to a drying technique suitable for drying a semiconductor wafer, and more particularly to improvement to prevent defective dryness and to reduce a size and cost of an apparatus.
2. Description of the Background Art
FIG. 16 is a sectional front view showing a structure of a drying apparatus according to the prior art which is the background of the present invention. A drying apparatus 151 is formed to dry a semiconductor wafer. The drying apparatus 151 comprises a processing vessel 171 having an upper end opened. A cooling coil 162 is attached to an inside of an upper portion of a side wall of the processing vessel 171 along the side wall of the processing vessel 171. The cooling coil 162 is formed of a silica tube, in which cooling water is caused to flow.
A heater 170 is provided just below a bottom portion of the processing vessel 171. Furthermore, a pan 166 is fixed to a position between the bottom portion and the upper open end in the processing vessel 171. A pipe 180 for draining the pan 166 is connected to a bottom portion of the pan 166. A fire extinguishing nozzle 172 is provided on an upper portion of the processing vessel 171. The fire extinguishing nozzle 172 serves to spout a fire extinguishing gas when a fire is caused in the processing vessel 171.
When using the drying apparatus 151, an IPA (isopropyl alcohol) solution 167 is first poured into the processing vessel 171. A depth of the processing vessel 171 is regulated in such a manner that a liquid level does not reach the bottom portion of the pan 166. The cooling water is caused to flow in the cooling coil 162.
When the heater 170 is turned on, the IPA solution 167 is heated. As a result, the IPA solution 167 is vaporized so that an IPA vapor 165 is generated. The IPA vapor 165 is filled into the processing vessel 171. The IPA vapor 165 is cooled and condenses in the vicinity of the cooling coil 162. More specifically, the cooling coil 162 serves to prevent the IPA vapor 165 from leaking out of the processing vessel 171.
Accordingly, the IPA solution 167 is stored in a liquid storing section 169 provided in the vicinity of the bottom portion of the processing vessel 171, and the IPA vapor 165 is filled into a vapor filling section 168 from a top of the IPA solution 167 to the vicinity of the cooling coil 162. After the IPA vapor 165 is filled into the vapor filling section 168, a processing is started for a semiconductor wafer 163 to be processed. After a rinsing processing is completed, a lot of semiconductor wafers 163 and a cassette 164 carrying them are suspended from a holding arm 161 and are inserted into the vapor filling section 168 from above the processing vessel 171. The cassette 164 carrying the semiconductor wafers 163 is held by the holding arm 161 just above the pan 166 as shown in FIG. 16.
Consequently, the IPA vapor 165 filled into the vapor filling section 168 condenses and dissolves into waterdrops which stick to surfaces of the semiconductor wafers 163 and the cassette 164. As a result, the waterdrops are essentially changed to IPA droplets. The IPA droplets slip from the surfaces of the semiconductor wafers 163 and the cassette 164. Thus, the semiconductor wafers 163 and the cassette 164 which are wet with the waterdrops can he dried. The slipping IPA droplets are collected by the pan 166 and discharged to an outside through the pipe 180.
When the drying processing is completed, the cassette 164 is pulled up by the holding arm 161 and is taken out of the processing vessel 171. Then, the cassette 164 thus taken out is delivered to a next processing step. Thereafter, new (i.e. unprocessed) semiconductor wafers 163 and a new cassette 164 are put into the processing vessel 171. Thus, the semiconductor wafers 163 and the cassette 164 are dried repeatedly.
In the drying apparatus 151 according to the prior art described above, the IPA vapor 165 is cooled by using the cooling coil 162 in which the cooling water flows, thereby preventing the IPA vapor 165 from leaking out of the processing vessel 171. For this reason, the IPA vapor 165 is sensitively affected by a temperature and flow of the cooling water in the cooling coil 162. More specifically, when conditions of the cooling water are changed, a state of the IPA vapor 165 such as a concentration of the vapor 165 or the extension of the vapor filling section 168 is varied.
When the state of the IPA vapor 165 is changed, an amount of the IPA vapor 165 condensing on the surfaces of the semiconductor wafers 163 and the cassette 164 is varied. As a result, defective dryness is sometimes caused, that is, the drying processing is not fully performed. Consequently, the yield of a semiconductor device manufactured in the semiconductor wafer 163 is deteriorated.
Furthermore, the processing vessel 171 requires an extra space for placing the cooling oil 162. Consequently, a size of the apparatus cannot be reduced. In order to prevent metallic contamination on the semiconductor wafer 163, the silica tube is used for the cooling coil 162. However, the silica tube is expensive so that a manufacturing cost of the apparatus is increased. In addition, the cooling coil 162 formed of the silica tube has a complicated structure. Therefore, it takes a long time and a high cost to repair the apparatus when troubles are made.